This invention relates to a thermal crystallization method of a neck of a primary molded product of a bottle-shaped container having a single layer or a multilayer including a gas barrier material. The present invention also relates to a jig to be used for the thermal crystallization method of the neck.
Bottles formed by biaxially oriented blow-molding of polyethylene terephthalate resin (to be referred to as PET resin hereinafter) have been made to show a small wall thickness in recent years for the purpose of cost reduction. Bottles having a thin wall are then required to show an effective gas barrier property in order to prolong the shelf life of the beverage contained in the bottle.
This requirement is currently met by using a primary molded product (to be referred to as “preform” hereinafter) having a multilayer structure of PET resin/gas barrier material/PET resin for molding a bottle, or by forming a primary molded product by injection molding of PET resin blended with a gas barrier material such as nylon (MXD-6) so as to provide the bottle, which is the secondary molded product formed from the preform by biaxially-oriented blow molding, with a gas barrier effect.
The preform (primary molded product) is formed by firstly injecting PET resin into a cavity through the nozzle of an injection machine whose injection pressure has been regulated to a predetermined level, then injecting a gas barrier material through the nozzle into the PET resin in the cavity, and subsequently injecting PET resin in the cavity under injection pressure that is also regulated to a predetermined level, in order to realize a multilayer structure.
The fluidity of the PET resin and the gas barrier material in the cavity can change as a function of a variety of factors including the temperature of the injection machine, the injection pressure, the injection rate, the difference of viscosity between the resin and the gas barrier material, the moisture content of the resin and the temperature of the manifold etc. Particularly, the fluidity is affected remarkably by temperature. Even if the temperature of the injection machine is defined rigorously, it is difficult to accurately control the temperatures of the resin and the gas barrier material in the injection machine. Thus, when molding the perform, part of the leading edge of the flowing gas barrier material is displaced toward the free end of the neck from the defined position on the circular periphery of the neck, while the remaining leading edge is displaced toward the body, because of a delicate change in the fluidity that is caused by fluctuations of various parameters including the injection pressure, the injection rate and temperature.
The neck of the preform produced by injection molding is a part that is not oriented during the secondary molding operation of biaxially-oriented blow-molding, so that it becomes thicker than the remaining parts of the bottle after the biaxially-oriented blow-molding. Therefore, that neck possesses a satisfactory gas barrier property, and hence it is not necessary to provide the neck with a gas barrier material layer.
For a bottle formed by biaxially-oriented blow-molding to show a satisfactory bas barrier effect, it is necessary that a gas barrier material layer is found at least in the shoulder and the body of the molded bottle. Therefore, a preform is prepared by placing the leading edge of the gas barrier material layer at the neck ring position of the neck when molding the preform. In other words, the preform is formed such that the gas barrier material is positioned at and below the neck ring.
However, as pointed out above, it is difficult to accurately control the position of the leading edge of the gas barrier material layer at the time of injection molding. Therefore, if the leading edge is defined to be positioned at the neck ring for the molding operation, the leading edge can be partially positioned at the bead ring that operates as functional part of the neck on the circular periphery of the neck. In other words, the leading edge can be positioned above the neck ring.
When a bottle formed by biaxially oriented blow-molding is used as heat-resistant bottle, it is necessary to whiten (thermally crystallize) the neck that is not drawn during the biaxially oriented blow-molding operation. When a preform having a multilayer structure of a PET resin layer and a gas barrier material layer is processed for thermal crystallization, the gas barrier material layer and the PET resin layer existing in the neck show respective coefficients of contraction that are different from each other due to the difference in the degree of crystallization, so that they do not contract exactly to the defined extent. Thus, there arises a problem that the neck is formed to show a diameter greater than the defined value.
As described above, the leading edge of the gas barrier material layer cannot be rigorously controlled for its position.
If the leading edge is tilted or incorrectly positioned on the circular periphery of the injection-molded neck, the neck is apt to be deformed to show an elliptic profile due to the difference in the coefficient of contraction caused by the difference in the degree of crystallization between the gas barrier material layer and the PET resin layer.
Particularly, if the functional part of the neck is not molded to the defined dimensions, problems such as a poor sealing effect and a poor screwing performance of the cap can arise. Additionally, even in the case of a bottle having a single layer of polyethylene terephthalate, there can arise an occasion where the neck does not show its proper profile due to the fluidity of resin during the injection molding process. In this case, the neck that is required to correctly show a circular profile may show an elliptic profile.
This invention is intended to dissolve the above identified problems of the prior art. It is the object of the present invention to provide a thermal crystallization method of the neck of the primary molded product of a biaxially-oriented blow-molded bottle having a single layer or a multilayer including a gas barrier material layer for improving the gas barrier effect of the bottle that is to be formed, wherein after the completion of the heating step, the bead ring is squeezed in the thermal crystallization process, to prevent the functional part of the neck from deforming, in order to make the functional part of the crystallized neck reliably show a satisfactory sealing effect. Another object of the present invention is to provide a jig to be used for the thermal crystallization method of the neck of the primary product.